Abstract Climate warming fosters an earlier spring green-up that may bring potential benefits toagricultural systems. However, advances in green-up timing may leave early stage vegetation growthvulnerable to cold damage when hard freezes follow green-up resulting in a false spring. Spatiotemporalpatterns of green-up dates, last spring freezes, and false springs were examined across the contiguousUnited States from 1920 to 2013. Results indicate widespread earlier green-up and last spring freeze dates overthe period. Observed changes in these dates were asymmetric with the last spring freeze date advancing toearlier in the year relative to green-up date. Although regionally variable, these changes resulted in a reductionin false springs, notably over the past 20 years, except across the intermountain western United States wherethe advance in green-up timing outpaced that of the last spring freeze. A sensitivity experiment shows thatobserved decreases in false springs are consistent with a warming climate.

Changes in the timing of plant and animal life cycle events, in response to climate change, are already happening across the globe. The impacts of these changes may affect biodiversity via disruption to mutualisms, trophic mismatches, invasions and population declines. To understand the nature, causes and consequences of changed, varied or static phenologies, new data resources and tools are being developed across the globe. The USA National Phenology Network is developing a long-term, multi-taxa phenological database, together with a customizable infrastructure, to support conservation and management needs. We explore applications of the infrastructure at the local level to inform invasive species management, and at the regional scale to understand gradients and cues for phenological response to climate. The approaches described here are congruent with recent trends towards multi-agency, large-scale research and action.

Recent warming of Northern Hemisphere (NH) land is well documented and typically greater in winter/spring than other seasons. Physical environment responses to warming have been reported, but not details of large-area temperate growing season impacts, or consequences for ecosystems and agriculture. To date, hemispheric-scale measurements of biospheric changes have been confined to remote sensing. However, these studies did not provide detailed data needed for many investigations. Here, we show that a suite of modeled and derived measures (produced from daily maximum{\textendash}minimum temperatures) linking plant development (phenology) with its basic climatic drivers provide a reliable and spatially extensive method for monitoring general impacts of global warming on the start of the growing season. Results are consistent with prior smaller area studies, confirming a nearly universal quicker onset of early spring warmth (spring indices (SI) first leaf date, -1.2 days decade-1), late spring warmth (SI first bloom date, -1.0 days decade-1; last spring day below 5{\textdegree}C, -1.4 days decade-1), and last spring freeze date (-1.5 days decade-1) across most temperate NH land regions over the 1955{\textendash}2002 period.However, dynamics differ among major continental areas with North American first leaf and last freeze date changes displaying a complex spatial relationship. Europe presents a spatial pattern of change, with western continental areas showing last freeze dates getting earlier faster, some central areas having last freeze and first leaf dates progressing at about the same pace, while in portions of Northern and Eastern Europe first leaf dates are getting earlier faster than last freeze dates. Across East Asia last freeze dates are getting earlier faster than first leaf dates.